The present invention generally relates to the field of liquid crystals. More specifically, the present invention relates to a liquid crystal device including a liquid crystal bulk layer, and a dynamic surface alignment layer interacting with the bulk layer for obtaining a preferred orientation of the surface director of the bulk layer. The invention also relates to a method for producing a liquid crystal device including such a dynamic surface alignment layer. Furthermore, the present invention relates to a method for accomplishing an in-plane switching in a liquid crystal bulk layer of a liquid crystal device.
The operation of almost all liquid crystal devices is based on a direct coupling between, on the one hand, an electric field applied in a perpendicular direction across a liquid crystal bulk layer within the device and, on the other hand, an induced polarization of the liquid crystal layer (in dielectric or paraelectric liquid crystals) or a spontaneous polarisation of the liquid crystal layer (in ferroelectric crystals). As a direct result of said direct coupling with the applied electric field, the orientation of the liquid crystal molecules within the bulk layer is changed, which in its turn results in an optical response so the device due to birefringent properties of the liquid crystal. The applied electric field will normally interact not only with the crystal molecules within the volume, but also with those molecules of the bulk layer that are located at the surface of the bulk layer. Typically, such interaction between the electric field and he surface molecules of the bulk layer may be less strong due to surface constraints.
There exists a number of different types of liquid crystal displays using liquid crystal devices, especially (1) Dynamic scattering displays; (2) Displays using deformation of homeotropically aligned nematics; (3) Schadt-Helfrich displays; (4) Supertwist-displays; (5) In-plane switching displays controlled by electric fields oriented in parallel with the substrates; and (6) Surface Stabilized Ferroelectric Liquid Crystal Displays (SSFLC Displays) and Displays with antiferroelectric liquid crystals.
For modern applications, a liquid crystal display should present several important properties, such as a low power consumption, a low threshold voltage, a steep electro-optical characteristic or bistability, a low viewing angle dependence of the contrast, short switching times, a high contrast, brightness, etc.
Today, some liquid crystal displays are advantageous concerning some of the above-mentioned desired properties, but there exists no ideal display which is optimized concerning all of the important properties.
Conventional nematic displays with a dielectric coupling to the electric field are usually slow, and nearly all suffer from non-satisfactory angular dependence of the contrast due to the out-of-plane switching of the liquid crystal molecules. The term xe2x80x9cout-of-plane switchingxe2x80x9d refers to the fact that the nematic liquid crystal molecules, when subjected to an external electric field, typically will tilt in relation to the plane in which the molecules normally are located in.
The dynamic scattering displays relies on strong movements of the molecules and inherently need relatively high electric field for switching and therefore, this display type is seldom used anymore. Displays with deformation of homeotropically aligned nematics as well as Schadt-Helfrich displays have a strong viewing angle dependence of the optical contrast, and the latter also possesses a low steepness of the electro-optical characteristic. Supertwist displays (twist angles of e.g. 270xc2x0) have an improved steepness of the electro-optical characteristic, but they present longer switching times and not yet satisfying viewing angle dependence of the optical contrast.
In contrast to the use of fields which are oriented perpendicular to the confining substrates, displays of the in-plane-switching type are controlled by electric fields oriented in parallelxe2x80x94not perpendicularxe2x80x94with the substrates. These displays possess very small dependence of the optical contrast from the viewing angle, but the brightness and switching times are not satisfying. A specific disadvantage of this display type is the requirement for an in-plane applied electric field which causes manufacturing problems.
Next, the surface stabilized ferroelectric liquid crystal devices (SSFLCs) will by considered, but first a short description of the nature of smectic liquid crystals will be given for a better understanding of the SSFLCs.
In a smectic liquid crystal, the molecules are arranged in adjacent smectic layers. Smectic A phase and smectic C phase are the two most important representatives of these xe2x80x9clayeredxe2x80x9d or smectic liquid crystals. In the C phase, the molecules are inclinded with an angle xcex2 (typically in the order of 22, 5xc2x0) with respect to the smectic layer normal, whereas in the A phase the molecules are perpendicular (xcex2=0xc2x0) to the smectic layers, i.e. directed along the smectic layer normal. Furthermore, a smectic liquid crystal can be non-chiral (e.g. A or C) or chiral (e.g. A* and C*), where the term chiral, means lack of mirror symmetry. It should be noted that the term xe2x80x9cchiralxe2x80x9d does not refer to the occurrence of a twisted or helical molecular arrangement that may or may not appear as a secondary effect as a result of the medium""s chirality.
A chiral smectic liquid crystal, such as C*, possesses a director that rotates in a cone in going from one smectic layer to the next. The apex angle xcex8=2xcex2 of the cone may typically be in the order of 45xc2x0. Thereby, a helix texture is formed across the layers with the helix axis being perpendicular to the smectic layers and parallel to the axis of said cone. However, the local polarisation which is coupled to the director will then also turn around in a helical fashion, with the same period or pitch. Such a helical structure of the local polarisation means that the local polarisation is self-cancelling, i.e. the bulk liquid crystal will present no macroscopic polarisation.
Now, if an electric field is applied parallel to the smectic layers in the helical smectic C* bulk state, the electric field will couple to the permanent dipoles and align them with the field direction. In other words, the applied field will unwind the helix and create an induced macroscopic polarisation of the bulk liquid crystal.
In a so-called SSFLC device, a chiral smectic liquid crystal is used (e.g. C*), but the above-mentioned helix is suppressed by the confining substrate surfaces and thereby not present. This is accomplished (i) by arranging the smectic layers non-parallel with the confining planes or substrates of the device (bookshelf or quasi-bookshelf structure), and (ii) by making the thickness of the smectic liquid crystal layer perpendicular to the substrates so small (in the order of microns) that the interaction of the liquid crystal molecules with the substrate surfaces produces a liquid crystal texture in which there is no longer any helical arrangement of the director within the cell. Instead, the liquid crystal molecules align in a predetermined direction, e.g. parallel, to the substrate despite the fact that a chirai material is used. Specifically, the director lying parallel with the substrates forms an angle (e.g. 22, 5xc2x0) to the smectic layer normal. Since the uniform surface conditions at the boundaries are in conflict with the helical bulk condition and, therefore, will quench the helix, the helix will be elastically untwisted by the boundaries when the cell thickness is chosen below a certain value. The result is what is called the surface-stabilized smectic C* state, presenting a non-zero macroscopic polarisation.
The material such as C* used in this device is in the ferroelectric phase, which means that, in the absence of an electric field, it presents a permanent polarisation along the smectic layers, i.e. perpendicular to the long molecule axis. Thereby, the director can be xe2x80x9cdigitallyxe2x80x9d switched between to states by reversal of the electric field applied in a direction perpendicular to the substrates. Such a cell is called a surface stabilized ferroelectric liquid crystal device (SSFLC device). An important feature is that the xe2x80x9cflipp-floppxe2x80x9d mechanism (xe2x80x9cthe Goldstone effectxe2x80x9d), which could also be bistable, is much faster than the rather slow dielectric mechanism that switches conventional LC materials having no permanent polarization, such as a conventional twisted nematic display.
In this context, it could be noted that the above-mentioned smectic layers are generally slightly tilted relative to the confining surfaces. Moreover, the may form a chevron structure rather than a bookshelf structure.
As an alternative Do using ferroelectric materials, the liquid crystal material can be an the so-called anti-ferroelectrical phase (AFLC), which means that in the absence of an electric field, the directors in adjacent smectic layers possess an opposite tilt. In a AFLC display, the polarizers are set parallel and perpendicular to the smectic layers. At E=0, a dark state is obtained, whereas +E and xe2x88x92E both give the same bright state. Thus, the AFLCs present a xe2x80x9cthree-state switchingxe2x80x9d, whereas the SSFLCs present a xe2x80x9ctwo-state switchingxe2x80x9d which could be bistable as well.
Drawbacks of the surface stabilized ferroelectric liquid crystal displays (SSFLCD""s) and the antiferroelectric liquid crystal displays include difficulties in orienting the smectic phase and maintaining the orientation of these phases. Also, the power consumption is relatively high. Another drawback is that the thickness of the liquid crystal layer must be limited, usually to about 2 xcexcm, in order to obtain the aimed-at coupling between an applied electric field and the spontaneous polarisation. This requirement for small thickness of FLC devices make their production complicated, delicate and expensive.
In the conventional liquid crystal devices discussed above, the desirable alignment of the liquid crystal within the bulk is achieved by appropriate treatment of the confining solid surfaces like coating with inorganic or organic layers as well as buffing (rubbing). In the absence of external fields, the initial liquid crystal alignment is defined by surface-liquid crystal interactions. Via steric interactions, the direction of the surface molecules will induce an orientation of the molecules within the bulk. For instance, near the substrate surface the director is constrained to point in a certain direction, such as perpendicular to the surface (homeotropic alignment) or parallel with the surface (planar alignment). Since the liquid crystal materials are strongly birefringent any change in their alignment will cause a certain change in their visible optical properties as seen between suitable polarizers.
In the prior art, there are in principle the following two different ways (1) and (2) for accomplishing a new molecule orientation different from the initial molecule alignment in order to change the optical properties:
(1) The first known technique for changing the molecule orientation is to apply an external field, such as an electric field over the entire liquid crystal bulk layer. Due to the above-mentioned direct coupling between the electric field and some of the liquid crystal material parameters within the bulk, the field directly interacts with the liquid crystal and changes the molecule alignment if the direction caused by the interaction differs from the preferred direction of the initial alignment. In some applications, the molecules near the surfaces are not very free to orient with the electric field, but the xe2x80x9cbulk moleculesxe2x80x9d more remote from the surfaces are fairly free to change its orientation.
(2) The second known technique for changing the orientation of the liquid crystal layer molecules is to design one of the confining alignment layers as a photo-controlled xe2x80x9ccommand surfacexe2x80x9d. When subjected to UV light, this command surface changes its aligning properties and thus changes the direction of the initial alignment of the liquid crystal interfacing with the alignment layer. This technique has been described by K. Ichimura et al. in two articles in Liquid Crystals, 1966, Vol. 20, No. 2, cages 423-435 and 161-169, respectively, and also in an Article entitled xe2x80x9cPhotocontrol of liquid crystal alignment by xe2x80x9ccommand surfacesxe2x80x9d xe2x80x9cin the documentxe2x80x9d Photoreactive Materials for Ultrahigh Density Optical Memoryxe2x80x9d, published by Institute of Advanced Material Study, Kyushu University, Japan.
More specifically, an azobenzene monolayer is deposited onto inner substrate surfaces of an LC cell including a nematic LC bulk layer. Conformational changes of the molecules in the alignment layer cause in its turn a change of the alignment of the nematic liquid crystal molecules. Specifically, the azobenzene molecules change their conformation from the xe2x80x9ctrans statexe2x80x9d to the xe2x80x9ccis statexe2x80x9d when irradiated with UV light. Photosensitive molecules, e.g. azocompounds, are anchored by the aid of triethoxysilyl groups at the substrate surfaces and are subjected to trans/cis-photoisomerization. The transisomer impose a homeotropic orientation of the nematic (nematic molecules perpendicular to the substrate), whereas the cis-isomer gives a planar one (nematic molecules parallel to the substrate). The photoisomerisation process take place when radiating the sample with UV light. The relaxation to the initial alignment is obtained by radiating the sample with white light or simply by heating it.
A drawback of using photocontrolled command layers in order to change the liquid crystal alignment is the slow switching speed between the two states. Especially, the transition from planar (cis) to homeotropic (trans) is a relatively slow process. Another drawback is that the life time of the device is reduced due to UV-light degradation. Furthermore, it can be used only for controlling nematic liquid crystals.
Therefore, today""s LC electrooptical displays are controlled not by light but by the use of external electric fields for changing the optical Properties of the bulk liquid crystal filled within the cell. Directional changes of the molecules can be made fast only by means of electric fields. The electric field directly interacts with the bulk liquid crystal and changes the orientation or alignment thereof, thereby changing properties of the display, such as light transparency, light absorption at different wavelengths, light scattering, birefringence, optical activity, circular dichroism, etc.
In view of the above-mentioned desired properties of a liquid crystal device, and the above-mentioned different drawbacks of the known displays, a general object of the present invention is to accomplish an improved liquid crystal display and a method for manufacturing such a display.
According to a first aspect of the invention (claim 1) there is provided a liquid crystal device presenting a dynamic surface alignment layer directly controllable by an electric field.
According to a second aspect of the invention (claim 36) there is provided a method for producing a liquid crystal device presenting a dynamic surface alignment layer directly controllable by an electric field.
According to a third aspect of the invention (claim 44) there is provided a method for accomplishing an induced in-plane switching in a liquid crystal bulk layer.
According to a fourth aspect of the invention (claim 45) there is provided a use of a liquid crystal material for producing dynamic alignment layer in a liquid crystal device.
Preferred and alternative embodiments of the invention are set out in the dependent claims.
According to the invention, the orientation of the surface molecules of the liquid crystal bulk layer are controlled by accomplishing an orientational change in one or two adjacent dynamic alignment layers. This orientational change in the dynamic alignment layer is accomplished by applying an electric field over the dynamic alignment layer in order to control the same. Normally, the electric field would also be applied across the LC bulk layer in addition to the dynamic alignment layer(s), but as far as the basic principle of the invention is concerned, any presence of the electric field over the LC bulk layer is not of primary importance, although in certain applications such presence may be useful.
In the present description of the inventive principle, the change of the dynamic alignment layer in response to the electric field is referred to as the xe2x80x9cprimary surface switchingxe2x80x9d. The primary surface switching results in its turn, via elastic forces (steric coupling), in a switching of the preferred molecule direction within the bulk volume of the liquid crystal material confined between the substrates. This secondary switching may be referred to as the xe2x80x9cinduced bulk switchingxe2x80x9d. This induced bulk switching is an in-plane switching. Thus, the change of the dynamic alignment layer causes a change of the surface molecules which are aligned by the alignment layer, and this change of the surface molecules accomplishes in its turn the induced bulk switching.
In a preferred embodiment of the invention, a chiral smectic liquid crystal material is used in order to form the electrically controllable dynamic alignment layer. Thus, according to the invention, it is possible to control or align a first liquid crystal layer (the bulk layer) by an electrically controlled, adjacent second liquid crystal layer (the dynamic alignment layer). Especially, this can be done without any need for direct coupling between the electric field and the liquid crystal material used for the bulk layer.
The above described inventive principle, as defined in the independent claims, gives rise to a number of very important and useful advantages over the prior-art liquid crystal displays. Especially, the following advantages 1-7 could be mentioned:
1. As a general advantage, tie invention allows fast switching times due to the use of electric fields for switching control.
2. The invention does not require any use of light, such as UV light, for accomplishing the switching function.
3. An important advantage of the invention is that is makes it possible to extend the advantages of in-plane molecule switching typical of e.g. SSFLCs, even including bistability, to conventional liquid crystal structures. As an example, a dynamic alignment layer according to the invention can be produced as a thin layer of smectic C* presenting a bistable, direct ferroelectric response to the applied electric field. A reversal of the field direction will result in a very quick response of the smectic C* layer. This the primary surface switching in response to the field reversal will be essentially as fast as the response of a conventional SSFLC. Due to (i) a coupling between the alignment layer and the surface molecules of the bulk layer, and (ii) a coupling between the surface molecules of the bulk layer and the molecules further into the bulk volume, the induced in-plane switching will be almost as fast as the primary surface switching. In other words, the invention makes it possible to electrically control a liquid crystal bulk layer in a very fast manner, without any specific requirement on the liquid crystal material, the thickness, the electrical response, etc of the bulk layer.
4. Many different types of liquid crystal materials can be used to implement the dynamic alignment layer.
5. Many different types of liquid crystal materials can be used to implement the bulk liquid crystal layer, such as a conventional non-chiral or chiral nematic or smectic in a twisted or non-twisted configuration.
6. The principle puts very few demands on the conventional liquid crystal and requires only a conventional and simple electrode structure.
7. The invention can be used to implement grey scale features. This can be performed by accomplishing a xe2x80x9ctwo-step effectxe2x80x9d, if the liquid crystal material of the bulk layer also presents a direct response to the applied electric field. As an example, one could have the bulk layer formed by a nematic liquid crystal having a small positive dielectric anisotropy, and having a threshold voltage for dielectric switching above that of the dynamic alignment layer. Now, when the electric field strength is increased, the first switching (the primary surface switching) occurs e.g. when the voltage reaches a first threshold voltage for dynamic alignment layer. At this point, there is still no direct electrical coupling between the electric field and the bulk molecules. However, there will be an induced in-plane switching of the bulk molecules.
This is the first step of the above-mentioned two-step effect. When increasing the field strength further, the voltage will exceed the threshold for dielectric switching within the bulk layer, leading to a homeotropic out-of-plane orientation of the nematic molecules. By the use of suitably orientated polarizers, the first step (induced in-plane switching) may extinguish light partly through the device, whereas the second step (the out-of-plane switching) may result I an essentially complete light blocking effect.
The aforementioned and other embodiments and advantages of the invention can be seen from the claims and the description hereinbelow of some embodiments of the inventive device and the manufacturing thereof.